There are numerous induction coil structures in the induction heating and baking fields as well as the field of plastic pipe fusion such as exemplified in the latter case by U.S. Pat. No. 5,125,690; International Publication Nos. WO81/02405 and WO82/02593 and WO82/02124 in addition to the related applications cited above. The general theme of all of these applications and the patent is to employ a coil to inductively heat an element disposed between plastic members to be joined so as to heat the element; a metallic material, preferably a material such as a ferromagnetic material, that has its magnetic permeability materially reduced as a function of an increase of temperature beyond a threshold temperature. The threshold temperature is one which closely approaches the effective Curie temperature of the material. Such material in the references cited above is located in each instance between the members to be joined.
Although the coils of the references are effective to some extent to accomplish the intended results, external radiation of magnetic flux is now recognized as a problem; particularly so as the knowledge of damage that can be caused by such external radiation increases.